I have a ’69 396 BB engine. The engine was totally rebuilt 10 years ago and I installed new rod bearings, rings, mild performance cam with timing gears, and ground the valves and seats. When the engine was fired up it ran like a fresh motor, not issues. It had a stock points ignition and a 4 barrel Qjet.
The engine then sat for 8 years due to unexpected issues in life. I rebuilt the carb to insure no stale gas/varnish issues. Upgraded to a new HEI distributor and new wires. I insured that the distributor was installed correctly ( #1 at TDC with rotor pointing to #1 tower). Fired it up and the vacuum gauge fluctuates rapidly between 11-14 inches. It occasionally backfires through the muffler. Adjusting the timing does not improve the symptoms. Any ideas on the root cause? Sticking valves?

Sticky valves could cause rough running but, my concern would be the vacuum, check for vacuum leaks, could be caused due to gaskets (intake gaskets) shrinking and leaking. Try running the engine and spraying a product like wd48 on the intake and head mating surface for example...if the vacuum gets more stable, you have found your problem.

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